Check if a string contains a valid postfix expression - c++

Question

I have a string that needs to be check for a valid postfix expression.
A valid postfix string is 1 2 + but not 1 2+ since each character needs a space. Also, since it's a string, you can enter words, but they should return -1 for this function.
I have tried using vector array with strings and checking for valid ints, but when the user enters letters this obviously creates a problem.
string postfix = "1 2 +"; // valid
string postfix = "soemthing"; // error
string postfix = "1 2+" ; // error since there is no space.
if (!isdigit(postfix[0]))
return -1;
int t;
string line = "55 124 4 5";
std::vector <int> ints;
std::istringstream iss ( line, std::istringstream::in);
int main() {
while (iss >> t )
{
ints.push_back(t);
}
if (!digit(ints[0]) || !digit(ints[0]))
return -1;
}
~

Answer 1

From this post you can get the algorithm to check. In c++:
int isValid(string postfix) {
int l = postfix.size();
char c;
bool numStarted = false;
int counter = 0;
for (int i = 0; i < l; i++) {
c = postfix.at(i);
if (numStarted == true && c == ' ') {
numStarted = false;
} else if (numStarted == false && c == ' ') {
return -1;
} else if (c == '-' || c == '+' || c == '*' || c == '/') {
if (counter < 2 || numStarted) {
return -1;
}
counter--;
numStarted = true;
} else if (!isdigit(c)) {
return -1;
} else if (!numStarted && isdigit(c)) {
counter++;
numStarted = true;
}
}
return (counter == 1 ? 1 : -1);
}
To test this:
int main(int argc, char** argv) {
string postfix1 = "1 2 +"; // valid
string postfix2 = "soemthing"; // error
string postfix3 = "1 2+"; // error since there is no space.
cout << isValid(postfix1) << endl;
cout << isValid(postfix2) << endl;
cout << isValid(postfix3) << endl;
return 0;
}
output:
1
-1
-1